Speed change gear devices are used as reduction gears or speed increasing gears that change the speed of rotation inputted by a driving source such as motor or the like. As the speed change gear devices like this, for example, Patent Documents 1 and 2 disclose reduction gears utilizing a planetary gear mechanism. In these reduction gears, planetary gears are revolved around a predetermined rotational axis, and only the rotational components of the planetary gears that components correspond to the difference in the number of teeth between external gears of the planetary gear and an internal gear meshing with the external gears are outputted through a transmission mechanism. With the construction like this, the reduction gears in Patent Documents 1 and 2 are deemed to be able to obtain a high speed change ratio through a single stage.